Incoherent X-ray scattering by K-shell electrons

Values of the incoherent scattering function S_K for scattering of photons from K-shell electrons are computed for ₂He, ₃Li, ₄Be and ₅B at photon energies 6, 10, 14, and 18 keV by using screened hydrogenic wavefunctions. The screening has been taken into account by the Thomas-Fermi model of the neutral atom. Unlike the previous calculations we have used a distorted Coulomb wavefunction for the ejected electron. The effects of Coulomb distortion and of screening corrections are discussed. It is shown that the screening correction and the Coulomb distortion in the ejected electron wavefunction play significant roles in the study of incoherent X-ray scattering by bound electrons.     

Schwinger-Lanczos approach to solving scattering equations

The Lanczos method, well known in linear algebra, is applied to solving Lippmann-Schwinger equation describing scattering of non-relativistic particles with atoms and molecules. It is shown that a combination of the Lanczos technique with an appropriate contracting coordinate transformation and Romberg extrapolation generates a very efficient and accurate method for calculation of various scattering quantities with a very sparse grid. The method yields accurate results for a very low number of meshpoints (typically few tens in electron atom scattering calculation). Detailed test calculations were performed for scattering of electrons with hydrogen atoms and for dissociative attachment of electrons to diatomic molecules.     

Bound state methods for elastic scattering phase shifts

Three methods are investigated for obtaining elastic phase shifts in the scattering of electrons from atoms and ions that use a bound-state representation of the scattering wavefunction. Results for singlet p-wave scattering by H and by He⁺ are compared with previous calculations.     

Proton and antiproton impact ionization of atomic hydrogen and helium

Single ionization of helium and atomic hydrogen by the impact of protons and antiprotons is considered. Using a multiple scattering model, first proposed by Garibotti and Miraglia [1], angular and energy distributions of the ejected electrons are calculated. Structures arising in the cross section, especially the Coulomb density of states effect (CDS), are analysed. The contributions of various scattering amplitudes to the cross section are studied. It is concluded that multiple scattering together with the CDS-effect play an important role in determining the transition amplitude. Differences between particle and antiparticle impact are examined. In addition to the different behaviour of the CDS-effect, the interference of two scattering amplitudes turns out to be decisive in ionization by particle and antiparticle impact.     

Delta-electron emission in fast heavy ion — atom collisions: observations of new phenomena and breakdown of common scaling laws

Double differential cross sections for the emission of Delta-electrons have been measured in fast uranium-rare gas collisions. The well-known Binary Encounter peak reveals unexpected structures for certain observation angles and its intensity increases towards smaller angles, which is in contradiction to results and scaling laws obtained by experiments with light ion impact. The observed dependencies are fairly well described by recent calculations in the framework of IA and CTMC. From systematic experimental as well as theoretical studies we can derive that the potential of the partially stripped projectile ion gives rise to rainbow and glory scattering of the target electron in the field of the projectile. The rainbow scattering is observed in the laboratory frame as pronounced interference structures, whereas the glory scattering is responsible for the steep increase of the cross sections for binary-encounter electrons towards small laboratory ejection angles. The observed effects have a dramatic influence on the commonq ² scaling laws derived from experiments with light ions. Furthermore, since the binary-encounter electrons ejected at forward angles have approximately twice the projectile velocity, these new phenomena have an important influence on the electronic stopping power of heavy ions and therefore have to be taken into account for the investigation of radiation damage by these ions e.g. in biological matter.     

Electron energy-loss spectra of acetaldehyde and formic acid

Apparent generalized oscillator strengths for acetaldehyde and formic acid molecules are derived, in the ranges 6.5–11.5 eV and 6.5–9.5 eV respectively, from energy-loss spectra of 150 eV incident electrons at scattering angles from 5 to 15 degrees. The functions show extrema, which is a characteristic of Rydberg transitions reported in some atoms and simpler molecules.     

Total (elastic + inelastic) cross sections for electron scattering with bound and free germanium and lead atoms

Spherical complex optical potential (SCOP) approach has been used to compute the differential, total (elastic + inelastic) and momentum transfer cross sections for electrons scattering from the bound and free germanium and lead atoms in the energy range from 100–5000 eV. We find that the present calculated differential scattering cross sections (DCS) exhibit all important features (such as forward peaking, dip at middle angles and enhanced backward scattering) observed in other theoretical calculations and experimental measurements. The effect of absorption potential is generally to reduce the elastic cross section.     

Use of spin-labelling techniques to probe the dynamics of He(23 S) deexcitation at solid surfaces

Spin labelling techniques, specifically the use of electron-spin-polarized He(2³ S) metastable atoms coupled with energy-resolved spin analysis of the ejected electrons, are used to investigate the dynamics of He(2³ S) deexcitation at solid surfaces. Data for a clean Au(100) surface are presented that show that deexcitation occurs exclusively through resonance ionization followed by Auger neutralization. The electrons involved in Auger neutralization are observed to be correlated in spin and possible reasons for this are discussed. Results obtained at Xe and NO films adsorbed on cooled Au(100) and Cu(100) substrates, respectively, show that He(2³ S) metastable atom deexcitation is analogous to gas-phase Penning ionization. Detailed differences are apparent that can be attributed to effects associated with the underlying substrate and interactions involving neighboring atoms in the film.     

Evaluation of elastic‐scattering cross sections for electrons and positrons over a wide energy range

Quantification of surface‐ and bulk‐analytical methods, e.g. Auger‐electron spectroscopy (AES), X‐ray photoelectron spectroscopy (XPS), electron‐probe microanalysis (EPMA), and analytical electron microscopy (AEM), requires knowledge of reliable elastic‐scattering cross sections for describing electron transport in solids. Cross sections for elastic scattering of electrons and positrons by atoms, ions, and molecules can be calculated with the recently developed code ELSEPA (Elastic Scattering of Electrons and Positrons by Atoms) for kinetic energies of the projectile from 10 eV to 50 eV. These calculations can be made after appropriate selection of the basic input parameters: electron‐density distribution, a model for the nuclear‐charge distribution, and a model for the electron‐exchange potential (the latter option applies only to scattering of electrons). Additionally, the correlation‐polarization potential and an imaginary absorption potential can be considered in the calculations. We report comparisons of calculated differential elastic‐scattering cross sections (DCSs) for silicon and gold at selected energies (500 eV, 5 keV, 30 keV) relevant to AES, XPS, EPMA, and AEM, and at 100 MeV as a limiting case. The DCSs for electrons and positrons differ considerably, particularly for medium‐ and high‐atomic‐number elements and for kinetic energies below about 5 keV. The DCSs for positrons are always monotonically decreasing functions of the scattering angle, while the DCSs for electrons have a diffraction‐like structure with several minima and maxima. A significant influence of the electron‐exchange correction is observed at 500 eV. The correlation‐polarization correction is significant for small scattering angles at 500 eV, while the absorption correction is important at energies below about 10 keV. Copyright © 2005 John Wiley & Sons, Ltd.     

Laser-assisted (e, 2e) collisions in helium

The influence of a strong laser field on the dynamics of fast (e, 2e) collisions in helium is analyzed in the asymmetric, coplanar geometry. The interaction of the laser field with the incident, scattered and ejected electrons is treated in a non-perturbative way, while the remaining interactions are treated by using first order perturbation theory. Detailed calculations are performed for an incident electron energyE _{k i}=600 eV, an ejected electron energyE _{k B}=5 eV and a scattering angle θ _A =4°. The influence of the laser parameters (photon energy, intensity and direction of polarization) on the angular distribution of the ejected electron is analyzed. We find that in general the triple differential cross sections are strongly dependent on the dressing of the projectile and the target by the laser field.     

Elucidating the initial dynamics of electron photodetachment from atoms in liquids using variably-time-delayed resonant multiphoton ionization

We study the photodetachment of electrons from sodium anions in room temperature liquid tetrahydrofuran (THF) using a new type of three-pulse pump-probe spectroscopy. Our experiments use two variably-time-delayed pulses for excitation in what is essentially a resonant 1+1 two-photon ionization: By varying the arrival time of the second excitation pulse, we can directly observe how solvent motions stabilize and trap the excited electron prior to electron detachment. Moreover, by varying the arrival times of the ionization (excitation) and probe pulses, we also can determine the fate of the photoionized electrons and the distance they are ejected from their parent Na atoms. We find that as solvent reorganization proceeds, the second excitation pulse becomes less effective at achieving photoionization, and that the solvent motions that stabilize the excited electron following the first excitation pulse occur over a time of approximately 450 fs. We also find that there is no spectroscopic evidence for significant solvent relaxation after detachment of the electron is complete. In combination with the results of previous experiments and molecular dynamics simulations, the data provide new insight into the role of the solvent in solution-phase electron detachment and charge-transfer-to-solvent reactions.     

Beyond the born approximation. The case of very long polymer chains adsorbed at an interface

Two experimental evidences are discussed of the reflectance discontinuity associated with very long adsorbed polymer chains. The anomalous low reflectivity is compared to the Ramsauer-Townsend effect in the scattering of slow electrons by rare-gas atoms.     

Spin polarization and cross sections of electrons elastically scattered from heavy alkaline-earth atoms

Semi-relativistic approach is employed to compute the differential and integrated cross sections, spin polarizationP and the spin polarization parametersT andU for the scattering of electrons from barium and strontium atoms in the energy range from 2.0–300 eV. The projectile-target interaction is represented both by real and complex optical potential in the solution of Dirac equation for the scattered electrons. The real optical potential includes the static, a parameter free correlation polarization and a modified semi classical exchange potentials. The complex optical potential is constructed by adding a model absorption potential as its imaginary part to the real optical potential.     

Interplay between collective and single electron excitations in large metal clusters

Collective and single electron excitations of large metal clusters supported on transparent substrates are investigated. The applied experimental techniques include extinction spectroscopy and laser induced dissociation accompanied by the ejection of individual atoms. The optical spectra depend on the electromagnetic far field and reflectcollective electron excitations of the conduction electrons, i.e. surface plasmons. Dissociation, however, is correlated to repulsivesingle electron energy levels. The characteristics of these localized excitations indicate a strong influence of collective excitations. In particular, it is found that nonlocal optical effects are important. In this picture surface plasmons catalytically enhance the number of single electron excitations and therefore of the metal atoms ejected as a result of the absorption of visible light. Results will be presented, which illustrate this interplay between collective and single electron excitations.     

Report on the 42nd IUVSTA workshop ‘Electron scattering in solids: from fundamental concepts to practical applications’

A summary is given of the workshop entitled ‘Electron Scattering in Solids: from fundamental concepts to practical applications,’ which was held in Debrecen, Hungary, on July 4–8, 2004, under the sponsorship of the International Union of Vacuum Science, Technique, and Applications (IUVSTA). This workshop was held to review the present status and level of understanding of electron‐scattering processes in solids, to identify issues of key importance (hot topics) in the light of the most recent scientific results, and to stimulate discussions leading to a deeper understanding and new solutions of current problems. This report contains summaries of presentations and discussions in sessions on elastic scattering of electrons by atoms and solids, inelastic scattering of electrons in solids, modeling of electron transport in solids and applications, and software. The principal areas of application include Auger‐electron spectroscopy (AES), X‐ray photoelectron spectroscopy, elastic‐peak electron spectroscopy (EPES), reflection electron energy‐loss spectroscopy (REELS), secondary‐electron microscopy, electron‐probe microanalysis (EPMA), and the use of coincidence techniques in electron‐scattering experiments. A major focus of the workshop was determination of the inelastic mean free path of electrons for various surface spectroscopies, particularly corrections for surface and core‐hole effects. Published in 2005 by John Wiley & Sons, Ltd.     

The generalized Sturmian method and inelastic scattering of fast electrons

The generalized Sturmian method for solving the many-electron Schrödinger equation is reviewed. This method yields rapidly convergent solutions directly, without the use of the SCF approximation. As a simple illustrative example, differential cross sections are calculated for inelastic scattering of fast electrons by atoms and ions in the 2-electron isoelectronic series.     

Generalized STU-parameters for elastic electron scattering from thallium and lead atoms

Numerical results for the complete set of generalized STU-parameters are presented for elastic scattering of polarized electrons from unpolarized thallium and lead atoms. Some of these parameters can only be measured in “triple scattering experiments”, since the change of the initial polarization vector has to be determined. The agreement between the predictions of semirelativistic Breit-PauliR-matrix calculations and recent experimental data of the Münster group for the Sherman functionS is excellent in the case of thallium while further improvements of the theoretical model are necessary for the lead target.     

Morphology,surface roughness,electron inelastic and quasielastic scattering in elastic peak electron spectroscopy of polymers

In elastic peak electron spectroscopy (EPES), the nearest vicinity of elastic peak in the low kinetic energy region reflects electron inelastic and quasielastic processes. Incident electrons produce surface excitations, inducing surface plasmons, with the corresponding loss peaks separated by 1–20 eV energy from the elastic peak. In this work, X‐ray photoelectron spectroscopy (XPS) and helium pycnometry are applied for determining surface atomic composition and bulk density, whereas atomic force microscopy (AFM) is applied for determining surface morphology and roughness. The component due to electron recoil on hydrogen atoms can be observed in EPES spectra for selected primary electron energies. Simulations of EPES predict a larger contribution of the hydrogen component than observed experimentally, where hydrogen deficiency is observed. Elastic peak intensity is influenced more strongly by surface morphology (roughness and porosity) than by surface excitations and quasielastic scattering of electrons by hydrogen atoms. Copyright © 2007 John Wiley & Sons, Ltd.     

Elastic Scattering of fast Electrons by Highly Polar Molecules

In this paper, peculiarities are considered of the angular dependence of the intensity of elastic scattering of fast electrons by dipolar LiH molecules, calculated in the isst Born approximation using Ransil's wavefunction. The molecular component of the scattering intensity is determined. lt is shown that the contribution of chemical bond effects to the intensity of electron scattering by molecules featuring highly polar bonds includes the part which is formally analogous to the structure dependent part of the intensity. Numerical integration of the Fourier transform of the molecular electron density was utilized to calculate the intensity of elastic electron scattering by LIH, LiF and LhO molecules, using HartreeùFock molecular wavefunctions. The major portion of the contribution of chemical bond effects to the intensity of electron scattering by the highly polar (ionic) LiF and Li20 molecules is made up by the ǒioniǒ contribution due to a redistribution of electrons between atoms making up an ionic molecule. A model is suggested of independent ions in a molecule, which correctly describes the “ionic” contribution of chemical bond effects to the intensity of electron scattering by highly polar molecules and is suitable for practical utilization for interpreting electron-diffraction patterns.     

Ultrafast charge-transfer-to-solvent dynamics of iodide in tetrahydrofuran. 2. Photoinduced electron transfer to counterions in solution

The excited states of atomic anions in liquids are bound only by the polarization of the surrounding solvent. Thus, the electron-detachment process following excitation to one of these solvent-bound states, known as charge-transfer-to-solvent (CTTS) states, provides a useful probe of solvent structure and dynamics. These transitions and subsequent relaxation dynamics also are influenced by other factors that alter the solution environment local to the CTTS anion, including the presence of cosolutes, cosolvents, and other ions. In this paper, we examine the ultrafast CTTS dynamics of iodide in liquid tetrahydrofuran (THF) with a particular focus on how the solvent dynamics and the CTTS electron-ejection process are altered in the presence of various counterions. In weakly polar solvents such as THF, iodide salts can be strongly ion-paired in solution; the steady-state UV-visible absorption spectroscopy of various iodide salts in liquid THF indicates that the degree of ion-pairing changes from strong to weak to none as the counterion is switched from Na+ to tetrabutylammonium (t-BA+) to crown-ether-complexed Na+, respectively. In our ultrafast experiments, we have excited the I- CTTS transition of these various iodide salts at 263 nm and probed the dynamics of the CTTS-detached electrons throughout the visible and near-IR. In the previous paper of this series (Bragg, A. E.; Schwartz, B. J. J. Phys. Chem. B 2008, 112, 483-494), we found that for "counterion-free" I- (obtained by complexing Na+ with a crown ether) the CTTS electrons were ejected approximately 6 nm from their partner iodine atoms, the result of significant nonadiabatic coupling between the CTTS excited state and extended electronic states supported by the naturally existing solvent cavities in liquid THF, which also serve as pre-existing electron traps. In contrast, for the highly ion-paired NaI/THF system, we find that approximately 90% of the CTTS electrons are "captured" by a nearby Na+ to form (Na+, e-)THF "tight-contact pairs" (TCPs), which are chemically and spectroscopically distinct from both solvated neutral sodium atoms and free solvated electrons. A simple kinetic model is able to reproduce the details of the electron capture process, with 63% of the electrons captured quickly in approximately 2.3 ps, 26% captured diffusively in approximately 63 ps, and the remaining 11% escaping out into the solution on subnanosecond time scales. We also find that the majority of the CTTS electrons are ejected to within 1 or 2 nm of the Na+. This demonstrates that the presence of the nearby cation biases the relocalization of CTTS-generated electrons from I- in THF, changing the nonadiabatic coupling to the extended, cavity-supported electronic states in THF to produce a much tighter distribution of electron-ejection distances. In the case of the more loosely ion-paired t-BA+-I-/THF system, we find that only 10-15% of the CTTS-ejected electrons associate with t-BA+ to form "loose-contact pairs" (LCPs), which are characterized by a much weaker interaction between the electron and cation than occurs in TCPs. The formation of (t-BA+, e-)THF LCPs is characterized by a Coulombically induced blue shift of the free eTHF- spectrum on a approximately 5-ps time scale. We argue that the weaker interaction between t-BA+ and the parent I- results in little change to the CTTS-ejection process, so that only those electrons that happen to localize in the vicinity of t-BA+ are captured to form LCPs. Finally, we interpret the correlation between electron capture yield and counterion-induced perturbation of the I- CTTS transition as arising from changes in the distribution of ion-pair separations with cation identity, and we discuss our results in the context of relevant solution conductivity measurements.